Position encoder

Claim 2

A position detector including: first and second members mounted for relative movement along a measuring path; said first member comprising a first circuit having at least two loops arranged in succession along said measuring path, each loop extending along said path and said loops being connected in series and being arranged so that EMFs induced in adjacent said loops by a common background alternating magnetic field oppose each other, and a second circuit extending along said measuring path; said second member comprising means for interacting with said circuits such that in response to an input driving signal applied to one of said circuits there is induced in the other said circuit an output signal, said interacting means and said at least two loops being arranged so that said output signal continuously varies as a function of their relative position along said path; characterized in that said interacting means comprises an electromagnetic resonant device arranged so that, in operation, an intermediate signal (Ir) is induced therein by said input driving signal applied to said one circuit and so that said intermediate signal induces said output signal in said other circuit.

Claim 3

A position detector according to claim 1, wherein said loops have a generally square shape.

Claim 4

A position detector according to claim 1, wherein said loops have a generally diamond shape.

Claim 5

A position detector according to claim 1, wherein said loops have a generally hexagonal shape.

Claim 6

A position detector according to claim 1, wherein said loops are constituted by opposed sinusoidal convolutions of conductor.

Claim 7

A position detector according to claim 1, wherein said first member comprises a plurality of said first circuits which are electrically separate from each other and are located one on the other, and wherein the plurality of loops of the respective first circuits are spatially separated along said path.

Claim 8

A position detector according to claim 6, wherein said first member comprises two of said first circuits, and wherein the loops of the respective first circuits are spatially separated along said path by half the extent of each of said loops.

Claim 9

A position detector according to claim 6, wherein said first member comprises three of said first circuits, and wherein the loops of the respective first circuits are spatially separated along said path by a third of the extent of each of said loops.

Claim 10

A position detector according to claim 6, wherein said first member comprises four of said first circuits, and wherein the loops of the respective first circuits are spatially separated along said path by a quarter of the extent of each of said loops.

Claim 11

A position detector according to claim 1, wherein said first circuit comprises more than two of said loops which are arranged in succession along said path.

Claim 12

A position detector according to claim 10, further comprising coarse positioning conductor means for identifying within which adjacent two loops of said first circuit said resonant device is located.

Claim 13

A position detector according to claim 10 or 11, wherein said first member comprises two of said first circuits, and wherein the extent of each loop forming one of said first circuits is greater than the extent of each loop forming the other of said first circuits.

Claim 14

A position detector according to claim 12, wherein said two first circuits are located on but electrically separate from each other.

Claim 15

A position detector according to claim 1, further comprising a second electromagnetic resonant device provided separate from and at a fixed distance along said path from the first electromagnetic resonant device, and wherein the spatial separation between the two electromagnetic resonant devices is such that the output signal induced in said other circuit by said first electromagnetic resonant device is in phase quadrature with the output signal induced in the other circuit by said second electromagnetic resonant device.

Claim 16

A position detector according to claim 1, comprising a plurality of said electromagnetic resonant devices, each having a different respective resonant frequency and being arranged such that in operation, each electromagnetic resonant device induces an alternating signal at the corresponding resonant frequency in said other circuit whose amplitude varies with its position relative to the loops of said first circuit.

Claim 17

A position detector according to claim 1, further comprising a drive means for applying said input driving signal to one of said first and second circuits and processing means for processing the signal induced in said other circuit and for providing an indication therefrom of the relative position of said first and second members.

Claim 18

A position detector according to claim 16, wherein said drive means is operable to apply said input drive signal to said second circuit.

Claim 19

A position detector according to claim 16, wherein said drive means is operable to apply said input drive signal to said first circuit, and wherein said second circuit comprises at least two loops arranged in succession along said path, each loop extending along said path and being arranged so that EMFs induced in adjacent said loops by a common background alternating magnetic field oppose each other.

Claim 20

A position detector according to claim 16, wherein said drive means is operable to apply a pulse of said driving signal during a first time interval and wherein said processing means is operable to process said induced signal during a subsequent second time interval after said first time interval.

Claim 21

A position detector according to claim 16, wherein said processing means comprises a demodulator which is matched to the phase of the signal induced in said other circuit by said resonant device.

Claim 22

A position detector according to claim 16, wherein said first member comprises two or more of said first circuits, wherein the loops of the respective first circuits are spatially separated along said path and wherein said processing circuit is operable to perform a trigonometric ratio calculation of the signals induced in said two or more first circuits.

Claim 23

A position detector according to claim 1, wherein said output signal induced in said other circuit varies sinusoidally as a function of the position along said path of said resonant device relative to said at least two loops, and wherein one period (T s ) of said sinusoidal variation corresponds to a relative movement of the extent (T s ) of two of said series connected loops.

Claim 24

A position detector according to claim 1, wherein said electromagnetic resonant device comprises a coil and a capacitor.

Claim 25

A position detector according to claim 1, wherein the loops of said first circuit are carried on a substantially planar surface of said first member.

Claim 26

A position detector according to claim 24, wherein said electromagnetic resonant device comprises a coil and a capacitor, and wherein the axis of the coil is generally perpendicular to said planar surface of said first member.

Claim 27

A position detector according to claim 25, wherein said first circuit comprises more than two of said loops which are arranged in succession over said path, and wherein said coil comprises a number of similar loops to those of said first circuit, which loops extend over the measurement path and are arranged such that when the resonant device is resonating, the output signal induced in said other circuit represents an average position of said resonant device relative to said first circuit over a number of the loops of said first circuit.

Claim 28

A position detector according to claim 25, wherein said resonant device comprises at least two series connected coils separated from one another in said path by a distance which can be varied to reduce the effect of tilt.

Claim 29

A position detector according to claim 1, wherein said resonant device comprises a coil (14), a capacitor (17) and a quartz or ceramic type resonator connected in series with said coil and capacitor.

Claim 30

A position detector according to claim 1, wherein said first member is fixed and wherein said second member is movable with respect to said first member.

Claim 31

A position detector according to claim 29, wherein said first and second circuits lie in substantially the same plane, and wherein said resonant device is movable relative to said first and second circuits in a plane which is substantially parallel to the plane in which said first and second circuits lie.

Claim 32

A position detector according to claim 1, wherein said measuring path is linear.

Claim 33

A position detector according to claim 1, wherein said measuring path is circular.

Claim 34

A position detector according to claim 1, wherein said first and second circuits are carried on a cylindrical surface of said first member.

Claim 35

A position detector according to claim 33, wherein said measuring path is linear.

Claim 36

A lift comprising a lift and a lift shaft and a position detector according to claim 1 for determining the position of the lift within the lift shaft.

Claim 37

A liquid level sensor comprising a float, a support on or in which the float is slidably guided and a position detector according to claim 1 for indicating the relative position of said float and said support.

Claim 38

A valve or throttle having a rotary shaft and a position detector according to claim 32 for determining the angular position of the shaft.

Claim 39

The valve or throttle of claim 37, wherein the angular travel of said shaft is less than 180°.

Claim 40

The valve or throttle of claim 37, wherein the angular travel of the shaft is not more than 120°.

Claim 41

A fluid flow meter comprising a tapered tube and a float in the tube that is displaced in a longitudinal position determined by the flow of fluid and a detector according to claim 1 for indicating the relative position of the float and the tube.

Claim 42

A fluid flow meter according to claim 40, wherein the float can rotate and carries a plurality of electromagnetic resonant devices, each comprising a coil and a capacitor, and wherein the axis of the coils of the plurality of resonant devices are orthogonal to each other.

Claim 43

A fluid flow meter according to claim 41, wherein the float can rotate and wherein two substantially planar first circuits are provided in mutually perpendicular planes.

Claim 46

A method according to claim 43, wherein said driving signal comprises an alternating signal having a frequency in the range of 10 kHz to 1 MHz.

Claim 47

A method according to claim 43, wherein said step of applying said driving signal applies a burst of said driving signal, and wherein said detecting step detects the signals induced in said other circuit after said burst of driving signal has been applied.

Claim 48

A position detector including: first and second members mounted for relative movement along a measuring path; said first member comprising a first circuit having at least two loops arranged in succession along said measuring path, each loop extending along said path and said loops being connected in series and being arranged so that EMFs induced in adjacent said loops by a common background alternating magnetic field oppose each other, and a second circuit extending along said measuring path; said second member comprising means for interacting with said circuits such that in response to an input alternating drive signal applied to one of said circuits there is induced in the other said circuit an output alternating signal which varies as a function of the position along said path of said interacting means relative to said at least two loops of said first circuit; characterized in that said interacting means comprises an electromagnetic harmonic generator arranged so that, in response to said alternating drive signal, an alternating output signal having a frequency different from that of the drive signal is induced in said other circuit.

Claim 49

A position detector including: first and second members mounted for relative movement along a measuring path; said first member comprising a first circuit having at least two loops arranged in succession along said measuring path, each loop extending along said path and said loops being connected in series and being arranged so that EMFs induced in adjacent said loops by a common background alternating magnetic field oppose each other, and a second circuit extending along said measuring path; said second member comprising means for interacting with said circuits such that in response to an input alternating drive signal applied to one of said circuits there is induced in the other said circuit an output alternating signal which varies as a function of the position along said path of said interacting means relative to said at least two loops of said first circuit; characterized in that said interacting means comprises an electromagnetic harmonic generator arranged so that, in response to said alternating drive signal, an alternating output signal having a frequency different from that of the drive signal is induced in said other circuit.

Claim 50

A position detector including: first and second members mounted for relative movement along a measuring path; said first member comprising a first circuit having at least two loops arranged in succession along said measuring path, each loop extending along said path and said loops being connected in series and being arranged so that EMFs induced in adjacent said loops by a common background alternating magnetic field oppose each other, and a second circuit extending along said measuring path; said second member comprising means for interacting with said circuits such that in response to an input driving signal applied to one of said first and second circuits there is induced in the other circuit an output signal which varies as a function of the position along said path of said interacting means relative to said at least two loops of said first circuit; characterized in that said interacting means comprises an electronic transponder arranged, in operation, to be powered by a magnetic field generated by said input driving signal applied to said one circuit and to induce said output signal in said other circuit, which output signal is distinguishable from the drive signal.

Claim 51

A position detector including: first and second members mounted for relative movement along a measuring path; said first member comprising a first circuit having at least two loops arranged in succession along said measuring path, each loop extending along said path and said loops being connected in series and being arranged so that EMFs induced in adjacent said loops by a common background alternating magnetic field oppose each other, and a second circuit extending along said measuring path; said second member comprising means for interacting with said circuits such that in response to an input driving signal applied to one of said first and second circuits there is induced in the other circuit an output signal which varies as a function of the position along said path of said interacting means relative to said at least two loops of said first circuit; characterized in that said interacting means comprises an electronic transponder arranged, in operation, to be powered by a magnetic field generated by said input driving signal applied to said one circuit and to induce said output signal in said other circuit, which output signal is distinguishable from the drive signal.

Claim 52

A position detector comprising: a sensing circuit for sensing alternating magnetic fields and having a sensing conductor lying substantially in a plane and extending over a measurement path; an energizing circuit, different from said sensing circuit, for generating an alternating energizing magnetic field, said energizing circuit having an energizing conductor lying in substantially the same plane as said sensing conductor; and a resonator electromagnetically coupled to said sensing circuit and said energizing circuit, at least one of the resonator and the sensing circuit being mounted for relative movement with respect to the other over said measurement path; said resonator being operable, upon energization by said excitation circuit at its resonant frequency, to resonate and to generate an alternating magnetic field which is substantially 90° out of phase with said energizing magnetic field, which resonator magnetic field induces an alternating signal in said sensing circuit which is similarly substantially 90° out of phase with any signal induced in said sensing circuit by said energizing magnetic field; said resonator and said sensing circuit being arranged so that the amount of electromagnetic coupling therebetween sinusoidally varies as a function of their relative positions, thereby causing the amplitude of said signal induced in said sensing circuit by said resonator magnetic field to vary in a similar sinusoidal manner as a function of the position of the resonator relative to the sensing circuit.

Claim 53

A position detector comprising: a sensing circuit for sensing alternating magnetic fields and having a sensing conductor lying substantially in a plane and extending over a measurement path; an energizing circuit, different from said sensing circuit, for generating an alternating energizing magnetic field, said energizing circuit having an energizing conductor lying in substantially the same plane as said sensing conductor; and a resonator electromagnetically coupled to said sensing circuit and said energizing circuit, at least one of the resonator and the sensing circuit being mounted for relative movement with respect to the other over said measurement path; said resonator being operable, upon energization by said excitation circuit at its resonant frequency, to resonate and to generate an alternating magnetic field which is substantially 90° out of phase with said energizing magnetic field, which resonator magnetic field induces an alternating signal in said sensing circuit which is similarly substantially 90° out of phase with any signal induced in said sensing circuit by said energizing magnetic field; said resonator and said sensing circuit being arranged so that the amount of electromagnetic coupling therebetween sinusoidally varies as a function of their relative positions, thereby causing the amplitude of said signal induced in said sensing circuit by said resonator magnetic field to vary in a similar sinusoidal manner as a function of the position of the resonator relative to the sensing circuit.

Claim 54

A position detector comprising: a sensor comprising first and second sensing circuits, each extending over a measurement path, said circuits being offset from each other in the measurement path and being operable for sensing alternating magnetic fields; an energizing circuit, different from said sensing circuits, for generating an energizing alternating magnetic field; a resonator which is electromagnetically coupled to said sensing circuits and said energizing circuit; at least one of said sensor and said resonator being mounted for relative movement with respect to the other over said measurement path; said resonator being operable, upon energization by said energizing circuit, to resonate and to generate an alternating magnetic field which induces an alternating signal in each of said sensing circuits; said resonator and said sensing circuits being arranged so that the amount of electromagnetic coupling therebetween varies sinusoidally as a function of their relative positions, thereby causing the amplitude of said signal induced in each of said sensing circuits by said resonator magnetic field to vary in a similar sinusoidal manner as a function of the position of the resonator relative to the corresponding sensing circuit; the amplitude of the signal induced in the first sensing circuit by the magnetic field generated by the resonator being different from the amplitude of the signal induced in the second sensing circuit by the magnetic field generated by the resonator, for a given relative position between the resonator and the sensor, due to the offset between the sensing circuits in the measurement path; and processing circuit connected to process and to demodulate the alternating signal induced in said first and second sensing circuits by the magnetic field generated by said resonator and to provide a trigonometric ratio of the demodulated signals from the respective first and second sensing circuits, which ratio is indicative of the relative position of said resonator and said first and second sensing circuits.

Claim 55

A position detector comprising: a sensor comprising first and second sensing circuits, each extending over a measurement path, said circuits being offset from each other in the measurement path and being operable for sensing alternating magnetic fields; an energizing circuit, different from said sensing circuits, for generating an energizing alternating magnetic field; a resonator which is electromagnetically coupled to said sensing circuits and said energizing circuit; at least one of said sensor and said resonator being mounted for relative movement with respect to the other over said measurement path; said resonator being operable, upon energization by said energizing circuit, to resonate and to generate an alternating magnetic field which induces an alternating signal in each of said sensing circuits; said resonator and said sensing circuits being arranged so that the amount of electromagnetic coupling therebetween varies sinusoidally as a function of their relative positions, thereby causing the amplitude of said signal induced in each of said sensing circuits by said resonator magnetic field to vary in a similar sinusoidal manner as a function of the position of the resonator relative to the corresponding sensing circuit; the amplitude of the signal induced in the first sensing circuit by the magnetic field generated by the resonator being different from the amplitude of the signal induced in the second sensing circuit by the magnetic field generated by the resonator, for a given relative position between the resonator and the sensor, due to the offset between the sensing circuits in the measurement path; and processing circuit connected to process and to demodulate the alternating signal induced in said first and second sensing circuits by the magnetic field generated by said resonator and to provide a trigonometric ratio of the demodulated signals from the respective first and second sensing circuits, which ratio is indicative of the relative position of said resonator and said first and second sensing circuits.

Claim 56

A position detector comprising: a sensing circuit extending over a measurement path for sensing alternating magnetic field oriented in a predetermined direction; an energizing circuit, different from said sensing circuit, for generating an energizing alternating magnetic field; and a resonator electromagnetically coupled to said sensing circuit and said energizing circuit; at least one of the resonator and the sensing circuit being mounted for relative movement with respect to the other over said measurement path in a plane that is substantially perpendicular to said predetermined direction; said resonator being operable, upon energization by said energizing circuit, to resonate and to generate an alternating magnetic field whose magnetic axis lies substantially in said predetermined direction, which resonator magnetic field induces an alternating signal in said sensing circuit; said resonator and said sensing circuit being arranged so that the amount of electromagnetic coupling therebetween varies sinusoidally as a function of their relative positions, thereby causing the amplitude of said signal induced in said sensing circuit by said resonator magnetic field to vary in a similar sinusoidal manner as a function of the position of the resonator relative to the sensing circuit.

Claim 57

A position detector comprising: a sensing circuit extending over a measurement path for sensing alternating magnetic field oriented in a predetermined direction; an energizing circuit, different from said sensing circuit, for generating an energizing alternating magnetic field; and a resonator electromagnetically coupled to said sensing circuit and said energizing circuit; at least one of the resonator and the sensing circuit being mounted for relative movement with respect to the other over said measurement path in a plane that is substantially perpendicular to said predetermined direction; said resonator being operable, upon energization by said energizing circuit, to resonate and to generate an alternating magnetic field whose magnetic axis lies substantially in said predetermined direction, which resonator magnetic field induces an alternating signal in said sensing circuit; said resonator and said sensing circuit being arranged so that the amount of electromagnetic coupling therebetween varies sinusoidally as a function of their relative positions, thereby causing the amplitude of said signal induced in said sensing circuit by said resonator magnetic field to vary in a similar sinusoidal manner as a function of the position of the resonator relative to the sensing circuit.

Claim 58

A position detector comprising: a sensing circuit for sensing alternating magnetic fields, said sensing circuit comprising a plurality of series connected conductive loops which extend successively over a measurement path and which are connected to each other so that any EMF induced in one of said loops by an alternating magnetic field is opposed by the EMF induced in an adjacent connected loop by the same magnetic field; an energizing circuit, different from said sensing circuit, for generating an energizing alternating magnetic field; and a resonator which is electromagnetically coupled to said sensing circuit and said energizing circuit; at least one of the resonator and the sensing circuit being mounted for relative movement with respect to the other over said measurement path; said resonator being operable, upon energization by said energizing circuit, to resonate and to generate an alternating magnetic field which induces an alternating signal in said sensing circuit; said resonator and said sensing circuit being arranged so that the amount of electromagnetic coupling therebetween varies sinusoidally as a function of their relative positions, the period of said sinusoidal variation corresponding to the extent, over said measurement path, of two of said series connected loops, thereby causing the amplitude of said signal induced in said sensing circuit by said resonator magnetic field to vary in a similar sinusoidal manner as a function of the position of the resonator relative to said sensing circuit; said resonator including a conductor having a number of loops similar to those of the sensing circuit, which loops extend along the measurement path and are arranged such that when the resonator is resonating, the signal induced in said sensing circuit represents an average position of said resonator relative to said sensing circuit over a number of said sensing circuit loops.

Claim 59

A position detector comprising: a sensing circuit for sensing alternating magnetic fields, said sensing circuit comprising a plurality of series connected conductive loops which extend successively over a measurement path and which are connected to each other so that any EMF induced in one of said loops by an alternating magnetic field is opposed by the EMF induced in an adjacent connected loop by the same magnetic field; an energizing circuit, different from said sensing circuit, for generating an energizing alternating magnetic field; and a resonator which is electromagnetically coupled to said sensing circuit and said energizing circuit; at least one of the resonator and the sensing circuit being mounted for relative movement with respect to the other over said measurement path; said resonator being operable, upon energization by said energizing circuit, to resonate and to generate an alternating magnetic field which induces an alternating signal in said sensing circuit; said resonator and said sensing circuit being arranged so that the amount of electromagnetic coupling therebetween varies sinusoidally as a function of their relative positions, the period of said sinusoidal variation corresponding to the extent, over said measurement path, of two of said series connected loops, thereby causing the amplitude of said signal induced in said sensing circuit by said resonator magnetic field to vary in a similar sinusoidal manner as a function of the position of the resonator relative to said sensing circuit; said resonator including a conductor having a number of loops similar to those of the sensing circuit, which loops extend along the measurement path and are arranged such that when the resonator is resonating, the signal induced in said sensing circuit represents an average position of said resonator relative to said sensing circuit over a number of said sensing circuit loops.

Claim 60

A position detector comprising: a sensing circuit for sensing alternating magnetic fields and having a sensing conductor lying substantially in a plane and extending over a measurement path; an energizing circuit, different from said sensing circuit, for generating an energizing alternating magnetic field; said energizing circuit having an energizing conductor which lies in a plane which is substantially parallel to the plane in which said sensing conductor lies and being operable to provide a pulse of said energizing alternating magnetic field during a first time interval; a self-contained electromagnetic device, electromagnetically coupled to said sensing circuit and said energizing circuit; at least one of the sensing circuit and the self-contained electromagnetic device being mounted for relative movement with respect to the other over said measurement path in a plane which is substantially parallel to the plane in which said sensing conductor lies; said self-contained electromagnetic device being operable, upon energization by energizing circuit, to generate an alternating magnetic field which is such as to induce an alternating signal in said sensing circuit; said self-contained electromagnetic device and said sensing circuit being arranged so that the magnetic coupling therebetween continuously varies as a function of their relative positions, thereby causing the amplitude of said induced signal to continuously vary with the position of the self-contained electromagnetic device relative to the sensing circuit; and processing circuitry operable to process, during a subsequent second time interval after said first time interval, the signal induced in said sensing circuit by said self-contained electromagnetic device and to provide the relative positions of said sensing circuit and electromagnetic device.

Claim 61

A position detector comprising: a sensing circuit for sensing alternating magnetic fields and having a sensing conductor lying substantially in a plane and extending over a measurement path; an energizing circuit, different from said sensing circuit, for generating an energizing alternating magnetic field; said energizing circuit having an energizing conductor which lies in a plane which is substantially parallel to the plane in which said sensing conductor lies and being operable to provide a pulse of said energizing alternating magnetic field during a first time interval; a self-contained electromagnetic device, electromagnetically coupled to said sensing circuit and said energizing circuit; at least one of the sensing circuit and the self-contained electromagnetic device being mounted for relative movement with respect to the other over said measurement path in a plane which is substantially parallel to the plane in which said sensing conductor lies; said self-contained electromagnetic device being operable, upon energization by energizing circuit, to generate an alternating magnetic field which is such as to induce an alternating signal in said sensing circuit; said self-contained electromagnetic device and said sensing circuit being arranged so that the magnetic coupling therebetween continuously varies as a function of their relative positions, thereby causing the amplitude of said induced signal to continuously vary with the position of the self-contained electromagnetic device relative to the sensing circuit; and processing circuitry operable to process, during a subsequent second time interval after said first time interval, the signal induced in said sensing circuit by said self-contained electromagnetic device and to provide the relative positions of said sensing circuit and electromagnetic device.

Claim 62

A position detector comprising: a sensing circuit for sensing alternating magnetic fields; an energizing circuit, different from said sensing circuit, for generating an energizing alternating magnetic field; at least one of said sensing circuit and said energizing circuit comprising a plurality of series connected conductive loops which extend successively over a measurement path and which are connected to each other so that any EMF induced in one of said loops by an alternating magnetic field is opposed by the EMF induced in an adjacent connected loop by the same magnetic field; and a harmonic generator electromagnetically coupled to said sensing circuit and said energizing circuit; said harmonic generator and said at least one of said sensing circuit and said energizing circuit being relatively movable over said measurement path; said harmonic generator being operable, upon energization by said energizing circuit, to generate an alternating magnetic field which is such as to induce harmonic signals in said sensing circuit; said harmonic generator and said sensing circuit being arranged so that the electromagnetic coupling therebetween varies sinusoidally as a function of their relative positions, the period of said sinusoidal variation corresponding to the extent, over said measurement path, of two of said series connected loops, thereby causing the amplitude of said induced harmonic signals to vary sinusoidally with the position of said harmonic generator relative to said sensing circuit.

Claim 63

A position detector comprising: a sensing circuit for sensing alternating magnetic fields; an energizing circuit, different from said sensing circuit, for generating an energizing alternating magnetic field; at least one of said sensing circuit and said energizing circuit comprising a plurality of series connected conductive loops which extend successively over a measurement path and which are connected to each other so that any EMF induced in one of said loops by an alternating magnetic field is opposed by the EMF induced in an adjacent connected loop by the same magnetic field; and a harmonic generator electromagnetically coupled to said sensing circuit and said energizing circuit; said harmonic generator and said at least one of said sensing circuit and said energizing circuit being relatively movable over said measurement path; said harmonic generator being operable, upon energization by said energizing circuit, to generate an alternating magnetic field which is such as to induce harmonic signals in said sensing circuit; said harmonic generator and said sensing circuit being arranged so that the electromagnetic coupling therebetween varies sinusoidally as a function of their relative positions, the period of said sinusoidal variation corresponding to the extent, over said measurement path, of two of said series connected loops, thereby causing the amplitude of said induced harmonic signals to vary sinusoidally with the position of said harmonic generator relative to said sensing circuit.